A current transformer is an apparatus for measuring a current flowing through a power system and inputting the measured current to a protective relay system. As shown in FIG. 1, the current transformer comprises a core 34 for concentrating a magnetic flux generated by a primary current flowing through a line 32 and a secondary coil 36 adapted to surround the core 34 for generating a secondary current using a magnetic flux induced to the core 34. A current flowing through the line 32 functioning as a primary coil induces a current proportional to the former current to the secondary coil 36, and the magnitude of current is determined according to a current transformation ratio. In this case, a steel core current transformer, in which the core 34 is steel, is chiefly used to maximize an interlinkage magnetic flux between the line 32 and the secondary coil 36.
FIG. 2 illustrates a schematic equivalent circuit of a current transformer. In this drawing, Lm denotes a magnetizing inductance, im denotes a magnetizing current, i1 denotes a secondary current (consistent with a current transformation ratio) induced to a secondary side by a primary current, and i2 denotes an actually measured secondary current. The magnetizing inductance Lm is not a constant, but has different values depending upon magnetizing currents. In particular, if a magnetic flux increases and exceeds a specific limit, the magnetizing inductance Lm varies considerably, which results from a variation in the internal state of a current transformer. In such a case, it is stated that the current transformer is saturated.
Since the magnitude of the magnetizing current im is small during normal operations, the measured primary current value of the current transformer is proportional to the primary current value thereof, so that a precise primary current value can be obtained from the measured secondary current value, thus causing no problem. However, if the magnetizing inductance value of the current transformer varies considerably by the saturation of the current transformer, the secondary current value varies considerably. If this phenomenon is described based on the equivalent circuit of FIG. 2, at the time of saturation, Lm value considerably decreases and, therefore, the magnetizing current im increases, so that the difference between the actually measured secondary current i2 and the secondary current i1 consistent with the current transformation ratio increases. Accordingly, a correlation between the actually measured secondary current i2 and the secondary current i1 becomes different after saturation. Meanwhile, the current transformer detects the value of a current flowing through the line using i2 even after saturation, so that it is imprecisely determined that the value of the current flowing through the line has decreased, thus causing the delay of the operation time of the protective relay system and the unwanted maloperation of the protective relay system.
FIGS. 6a and 6b are examples of magnetization curves showing correlations between magnetizing currents and interlinkage fluxes before and after saturation. FIG. 6a shows the transition of a magnetization curve in unsaturated and saturated regions. The slope of the magnetization curve represents magnetizing inductances Lm. FIG. 6b shows an example of an actual magnetization curve. As shown in FIGS. 6a and 6b, magnetizing inductances are considerably different before and after saturation.
For a conventional technology of compensating for current distortion resulting from the saturation of a current transformer, which is a main reason for the unwanted maloperation of a protective relay system, and obtaining a secondary current consistent with an actual current transformation ratio, there is proposed a method of calculating a magnetic flux in a steel core constituting part of a current transformer at the time of saturation and compensating for the distorted secondary current using the calculated magnetic flux to obtain a secondary current consistent with a current transformation ratio. However, the conventional method can be applied only to the case where a remanent magnetic flux does not exist at an early stage. In the case where a remanent magnetic flux exists at an early stage, the application of the conventional method is limited, if the initial value of the remanent magnetic flux is not known. In most applications, it is difficult to measure and estimate the value of the remanent magnetic flux using existing technology, so that the above-described disadvantage becomes fatal.